1. Field of the Invention
This invention relates generally to ADSL hybrid networks, and more particularly to a method of providing a switchable hybrid design suitable for ADSL applications.
2. Description of the Prior Art
The goal of the hybrid network is to model the transmit echo path transfer function to perform analog echo cancellation. Using a passive termination line driver, the transmit echo transfer function is given by
            H      Echo        ⁡          (      f      )        =                    Z        L        ′            ⁡              (        f        )                            R        T            +                        Z          L          ′                ⁡                  (          f          )                    where Z′L is the line impedance as seen from the receive terminals (modem side) of the transformer. Denoting the hybrid transfer function as HHyb, the hybrid echo rejection is defined asHEcho Rejection(f)=Hecho(f)−HHyb(f)
It is well known that the line impedance, and therefore Z′L, varies a great deal for different loop topologies, in particular in the presence of bridge-taps. FIGS. 1a and 1b illustrate the severity of the problem in which real and imaginary parts of the impedance are shown for straight loops (FIG. 1a) and some bridge tap loops (FIG. 1b) respectively.
It is clear that a single hybrid network cannot achieve the same cancellation levels for all possible loop scenarios. It would therefore be advantageous in view of the foregoing to provide an adaptive hybrid network capable of achieving substantially the same cancellation levels for all possible loop scenarios associated with a particular application.